The principles of multicarrier modulation are described for example in "Multicarrier Modulation For Data Transmission: An Idea Whose Time Has Come" by John A. C. Bingham, IEEE Communications Magazine, Vol. 28, No. 5, pages 5-14, May 1990. As is known, in a transmission system using multicarrier modulation, FDM (frequency division multiplexed) carriers spaced within a usable frequency band of a transmission channel, forming a set of subchannels, are modulated at a block or symbol transmission rate of the system. The bits of input data for transmission within each block or symbol period are allocated to the carriers or subchannels in a manner which is dependent upon the signal-to-noise ratios (SNRs) of the carriers or subchannels, typically so that the bit error rates of the subchannels, as monitored at the receiver, are substantially equal. As a result, the different subchannels carry different numbers of bits in each symbol period. With an appropriate allocation of bits and transmit powers to the carriers or subchannels, such a system provides a desirable performance.
Over a period of time during normal operation of a transmission system using multicarrier modulation, especially with changes in temperature and traffic changes on adjacent transmission channels, the SNRs of the carriers or subchannels will vary. In consequence, regardless of how the allocation of bits to carriers is initially established, it is desirable to update the allocations during operation of the system. This invention is concerned with such updating.
In the Chow et al. application referred to above, the receiver monitors a parameter such as the SNR of each subchannel and, in dependence upon the results of such monitoring, sends (via one direction of an operations control channel) bit swap information to the transmitter when it determines that a change in bit allocations is desirable. The transmitter sends (via an opposite direction of the operations control channel) an acknowledgement of the bit swap information back to the receiver, and it is proposed that the change in the bit allocations take place a fixed number of transmitted symbols following the acknowledgement. While this proposed so-called handshaking procedure is simple, it has disadvantages in practice which render it unreliable. In particular, errors in the handshaking procedure can cause the bit allocations to be changed on the occurrence of different symbols at the transmitter and the receiver, or to be changed at the transmitter and not at the receiver, or vice versa, leading to large numbers of errors in the transmitted data.
As the operation of the multicarrier modulation system is based on the allocation of bits to carders being coordinated between the transmitter and the receiver, it is essential that a reliable procedure be used for updating of the allocation of bits to carders.
An application of a transmission system using multicarrier modulation is an ADSL (asymmetric digital subscriber line) arrangement. For example, an article by S. Fleming et at. entitled "ADSL: The on-ramp to the information highway", Telephony, Jul. 12, 1993, pages 20-26 describes an ADSL arrangement applied to a two-wire telephone subscriber line, in which four asymmetric 1.5 Mb/s channels are provided for transmission in a downstream direction from a telephone CO (central office) to a subscriber, in addition to various data channels and POTS (plain old telephone service) carded symmetrically (i.e. bidirectionally) on the fine. The data channels for example comprise an ISDN (integrated services digital network) HO channel at 384 kb/s or an ISDN basic access channel at 144 kb/s, and a control channel for example at a bit rate of 16 kb/s. The asymmetric channels provide a total bandwidth of 6 Mb/s which can be used for digital video signals. These channels are in addition to the bidirectional operations control channel referred to above for communications between the transmitters and receivers of the system.
An object of this invention is to provide an improved method of updating allocations of bits to carriers in a transmission system using multicarrier modulation.